The AM Forum

I am in the process of bringing up my "new" home brew transmitter.  Yesterday I powered up the modulator power supply and it behaved reasonably.  It has a multi tap transformer connected to a rotary switch, I can select five voltages from 1KV to about 1.6KV. 

AFTER I tested it I noticed that the rating for the two oil filled caps are 10uF 1000V.

Can I expect this voltage to drop below 1KV under load?  (This rig is a pair of 811A's modulating a pair of 813's)

How dangerous is it to put such an over-voltage on the caps for short periods of time?

Thanks

Rich

Do not over voltage caps, not safe

Are the caps connected in series by any chance? That would give you 5 uf at 2000 volts and would be fine. It is odd that the power supply could produce up to 1600 volts and the caps are rated at only 1000 unless they are in series.

I don't think they are in series....  One end of each cap goes to ground.  The output circuit is a choke, cap to ground, a choke, and then the second cap to ground.  Or perhaps it is cap, choke, cap, choke, I'll have to check.

Rich

Be glad you got away with it once, but I wouldn't risk it again.

Do you know what your unloaded voltage is when strapped for 1k? If it's around 1k, you may luck out and load down to the 850-900 range, but even that's playing it close.

If they were 20 @ 1000, you could stack them, but 5uF probably won't cut it for you.

Interesting output circuit, why the combination of cap & choke output filtering?

--Thom
Killer Agony One Zipper Got Caught

Oil caps are typically rated at DC Working Volts...so considering peak ratings, you probably have a little margin over the rating for safety factor, but not a good idea to run them at full ratings.  When they blow...they can blow explosively!  Kinda scares the heck outa ya, I know...I have witnessed such things.

73,  Jack, W9GT

I will have to check through my notes but I believe the original QST article using "modified" 1625's in grounded grid used an oil filled cap at above its voltage rating and I wonder if this is where your builder got the idea.  I have one of these amp's and it came with a copy of the article it was built from but I decided to change the filter cap before I tried it.  I believe the early P&H amps were inspired by this article.  In any case, I wouldn't want to use the cap at above its rated voltage.

Rodger WQ9E

I will have to measure the voltages under load.  Also, once they are back in the rack, things will be a little more explosion proof.   ;-)

Rich

In his April 1954 QST article, "The Case for the AB1 Linear", George Grammer W1DF discusses using a 1KV rated oil filled cap in a circuit where the voltage could rise to 1700 volts if the bleed current drops and states that typically an oil filled cap is tested to a voltage of at least 1000 volts over its rating in a condenser of the size used. (in this case it is a 10 uf, 1KV rated unit).  Note that he does not actually advise using a capacitor at this sort of over voltage but I imagine a "hopeful builder" might have taken this note as being advice to do so.

I picked up a nice homebrew version of the amplifier built from this article along with two spare sets of modified 1625 tubes for $5 at a hamfest about 5 years ago.  In my case, the transformer used provides a DC output of just under 1,200 volts.  I had a 1500 volt capacitor which I subbed for the one in the unit and the amplifier does work.  I will probably hook it up to my CE-20A someday and give it a try.

Rodger WQ9E

the real danger is that the bleeder will fail and the joltage will soar as a result. Build everything for double the trouble and you will have none.

Wise words from an experienced builder!

There is no pressure relief in these old caps so be ready when they explode.
Do not use the builders stupidity as an excuse to take the risk of blowing up something that can hurt you. As Derb said derate properly and it will live for ever.
Danger Will Ribinson

mack,

it's just not worth it. Oil filled caps at the voltages you need are as cheap as New Orleans hoes. Hell, I bet some on here will send you some for free just for asking. I would, but I gave all my stuff away or threw it away or ebay'ed it long ago.

Orrr, I could design a simple PCB that would mount vertically in place of the oil filled caps.  The PCB would contain modern electrolytics in series.  It is my understanding that caps in series require a "load" resistor across each one.  Anyone know how to pick a proper value resistor?

Rich

My advice is don't over voltage the oil filled caps.....But if you have iron chokes in the filter section you don't need a lot of MF to get a ripple free dc output.   There are those that like a massive cap (30-60 MFD) with no chokes and I agree that this works....There are also those that go for the choke cap combo with smaller value(MFD) caps......I agree that this works as well...I have built with both systems and find that either works well......Go with what parts you have...that is what  I do....just don't over voltage those caps... I have blown them up and it ain't pretty.....most will take it for a while but sooner or later they will pop....good luck   Steve

I would keep at least a 20% safety margin on working voltage.  I have had an oil cap to fail only once or twice, and it merely shorted out and blew a fuse, no other harm done, but no point in taking a chance.  OTOH I have seen electrolytics literally blow up like a firecracker, scattering stinky paper-looking residue all over the place.

If the bleeder went out and the power supply accidentally lost load, or intentionally as in the case of CW, the voltage might soar more than 40% above normal, but hopefully that would be discovered right away.  I think most oil caps can take a substantial over-voltage for a short duration on a rare occasion without a problem, but I wouldn't count on that over a normal duty cycle.

With voltage rating, it's like anything else,

Better to have it and not need it, than to need it and not have it.